A number of new battery chemistries are entering the market to provide capabilities required in specialized applications. At one time, the lithium-ion battery market was driven by the use of such batteries in portable electronics which require high energy but only limited life and power. More recently, other industries have focused on the use of batteries. By way of example, batteries are commonly incorporated into power tools and certain types of hybrid-electric vehicles. Each new industry requires different performance characteristics. Certain applications such as automotive applications require battery stability both in terms of battery safety for large packs and long life (at least 10 to 15 years).
Over the life of electrochemical cells, capacity can be lost due to side reactions and/or active material loss. Cell capacity is defined herein as the capacity of the cell between two particular cell voltages (e.g., 2.8 V and 4.1 V). The change in capacity usually results in changes in the relationship between the open cell potential (OCP) and capacity of the cell. The change in the relationship becomes problematic in that the OCP of a cell is a convenient measure of the state of charge (SOC) of the cell so long as the relationship between the OCP and the capacity of the cell is known.
A full discharge of a cell can be used to periodically establish the relationship between the OCP of a cell and the SOC of the cell. The opportunity to fully discharge a cell, however, is not readily available for various cell usages. By way of example, the entire available capacity range of the battery is not accessed during normal operation of a hybrid-electric vehicle. Moreover, it is inconvenient to interrupt operation of a vehicle merely to fully discharge a battery in order to diagnose the remaining capacity of the battery cells.
An article, Bloom, et al., “Differential voltage analyses of high-power, lithium-ion cells: 1. Technique and application,” Journal of Power Sources, 139 (2005) 295, discusses a technique wherein low-rate measurements of cell voltage as a function of capacity could be used to determine the dominant contributors to capacity fade in a battery. The technique involves differentiation of the discharge voltage-vs.-capacity curves of a cell in order to obtain recognizable signatures that indicate the relative states of charge of the positive and negative electrodes. This technique, however, is limited in precision.
What is needed therefore is a method of determining the extent and type of capacity fade in a dual intercalation battery (e.g., a lithium-ion battery). A further need exists for a method which allows determination of the capacity of a battery when only a limited capacity or state-of-charge range is sampled. Additionally, the ability to more precisely identify features contributed by individual electrodes would be beneficial.